1. Field of the Invention
The present invention relates to a balanced-type surface acoustic wave filter provided with a balance-unbalance conversion function, and more particularly, to a balanced-type surface acoustic wave filter in which an IDT has a narrow pitch electrode finger section and an input/output impedance ratio is set to a specific value.
2. Description of the Related Art
In the related art, a balanced-type surface acoustic wave filter provided with a balance-unbalance conversion function is widely used in communications equipment, such as mobile phones. For example, Japanese Unexamined Patent Application Publication No. 2001-308672 (Patent Document 1) described below discloses a balanced-type surface acoustic wave filter, which is shown in FIG. 26.
In a balanced-type surface acoustic wave filter 501 shown in FIG. 26, longitudinally coupled resonator type surface acoustic wave filters 503 and 504 are connected to an unbalanced input terminal 502. The surface acoustic wave filters 503 and 504 include first to third IDTs (interdigital transducers) 503a to 503c and 504a to 504c arranged in the surface acoustic wave propagating direction, respectively. The second IDTs 503b and 504b in the center are electrically connected to the unbalanced input terminal 502. In the surface acoustic wave propagating direction, the first and third IDTs 503a and 503c arranged on both the sides of the IDT 503b are electrically connected to a first balanced output terminal 505, and the first and third IDTs 504a and 504c arranged on both the sides of the IDT 504b in the center of the surface acoustic wave filter 504 are electrically connected to a second balanced output terminal 506.
In the balanced-type surface acoustic wave filter 501, the input impedance on the unbalanced input terminal 502 side is set to 50 Ω, and characteristic impedances on the sides of the first and second balanced output terminals 505 and 506 are set to 150 Ω. That is, the input/output impedance ratio is set to 1:3. This is because an antenna is connected to an input terminal of the balanced-type surface acoustic wave filter 501 and the impedance is usually set to 50 Ω, whereas the input impedance of an IC connected to the output side is set to 150Ω.
Japanese Unexamined Patent Application Publication No. 6-204781 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 11-97966 (Patent Document 3) described below respectively disclose balanced-type surface acoustic wave filters provided with a balance-unbalance conversion function similar to the balanced-type surface acoustic wave filter described in Patent Document 1, but having different electrode configurations. In addition, in the balanced-type surface acoustic wave filters described in Patent Documents 2 and 3, the impedance ratio between the unbalanced signal terminal and the balanced signal terminal is set to about 1:3 to about 1:4.
On the other hand, in recent years, a balanced-type mixer IC with an input impedance of about 100 Ω is commercially available as an IC to be connected to the output side of the balanced-type surface acoustic wave filter of this kind. To cope with such an IC, the output impedance of the balanced-type surface acoustic wave filter connected to the IC must be set to about 100 Ω. Then, to set the impedance on the input terminal side to 50 Ω, an input/output impedance ratio of the surface acoustic wave filter is required to be 1:2.
Japanese Unexamined Patent Application Publication No. 2004-48675 (Patent Document 4) described below discloses a balanced-type surface acoustic wave filter provided with a balance-unbalance conversion function in which the impedance ratio between an unbalanced signal terminal and a balanced signal terminal is set to 1:2. FIG. 27 is a plan view showing the electrode configuration of the balanced-type surface acoustic wave filter described in Patent Document 4. In a balanced-type surface acoustic wave filter 601, connected to an unbalanced input terminal 602 are longitudinally coupled resonator type surface acoustic wave filter sections 603 and 604. The longitudinally coupled resonator type surface acoustic wave filter sections 603 and 604 respectively include first to third IDTs 603a to 603c and 604a to 604c arranged in the surface acoustic wave propagating direction. The IDTs 603b and 604b in the center are electrically connected to the unbalanced input terminal 602. The first and third IDTs 603a and 603c located on both the sides of the surface acoustic wave propagating direction of the IDT 603b are electrically connected to a first balanced output terminal 605. In the same manner, in surface acoustic wave filter section 604, the IDTs 604a, 604c located on both the sides of the IDT 604b in the surface acoustic wave propagating direction are electrically connected to the second balanced output terminals 606.
Here, the IDTs 603a and 603b include narrow pitch electrode finger sections N in the neighborhood of the areas adjacent to each other with a gap interposed therebetween in the surface acoustic wave propagating direction. That is, the electrode finger pitch of the area of IDT 603a near the IDT 603b has a narrower electrode finger pitch as compared to the rest of the areas. This IDT area where the electrode finger pitch is relatively narrow is referred to as a narrow pitch electrode finger section N. In the same manner, the area of IDT 603b near the IDT 603a at the side end is provided with a narrow pitch electrode finger section N. Furthermore, in the IDTs 603b and 603c and IDTs 604a to 604c as well, the respective neighborhoods of the adjacent areas with the gap are provided with the narrow pitch electrode finger section N.
Patent Document 4 also describes that in the balanced-type surface acoustic wave filter 601, the IDTs 603b and 604b on the unbalanced side and the IDTs 603a, 603c, 604a, and 604c on the balanced side are set to have different numbers of the electrode fingers, whereby the input/output impedance ratio can be set to 1:2.
In the surface acoustic wave filter, the impedance of the IDT decreases as an electrode finger cross width is increased and the number of electrode fingers is increased. In the surface acoustic wave filter 601 shown in FIG. 25, for example, the IDT 603b in the center of the surface acoustic wave filter section 603 is connected to the unbalanced input terminal 602, the IDTs 603a and 603c on both the sides are connected to the first balanced output terminal 605.
Therefore, to adjust the impedance ratio between the unbalanced signal terminal and the balanced signal terminal, the cross width of the IDT 603b and the cross widths of the IDTs 603a and 603c cannot be different from one another. Therefore, as described above, the number of the electrode fingers of the IDT 603b on the unbalanced side and the numbers of the electrode fingers of the IDTs 603a and 603c on the balanced side are made different from each other, and the input/output impedance ratio is changed. For example, the impedance of the first balanced output terminal is a value depending on the number of the electrode fingers of the IDT 603a on the balanced side. This is because the IDTs 603a and 603c are connected in parallel with respect to a balanced output terminal 605 and at the same time the balanced output terminal 605 and the balanced output terminal 606 are connected in series via a ground potential. In contrast to this, the impedance of the unbalanced terminal 602 side has a value corresponding to ½ of the number of the electrode fingers of the IDT 603b on the unbalanced side. That is, when the number of the electrode fingers of the IDT 603b on the unbalanced side is set to ½ of the number of the electrode fingers of the IDT 603a on the balanced side, the impedance ratio between the unbalanced signal terminal and the balanced signal terminal can be set to 1:2
However, the ratio between the numbers of the IDT 603b and 604b on the unbalanced side and the numbers of the IDTs 603a, 603c, 604a, and 604c on both sides thereof is changed, and a problem occurs in that an influence is imparted to resonant frequencies of a plurality of resonant modes for obtaining a pass band of the surface acoustic wave filter 601. Thus, the number ratio cannot be significantly changed. Since the number of the IDT on the unbalanced side is much less than the numbers of the IDTs 603a and 603c on the balanced side, there is also a problem in that a sufficient band width of a filter cannot be obtained.
In addition, with a method of changing the ratio between the numbers of the electrode fingers, the impedance ratio between the unbalanced signal terminal and the balanced signal terminal cannot be precisely set to 1:2, and the set ratio is often shifted from 1:2.